title: Population shift underlies Ca2+-induced regulatory transitions in the sodium-calcium exchanger (NCX).
abstract: In eukaryotic Na(+)/Ca(2+) exchangers (NCX) the Ca(2+) binding CBD1 and CBD2 domains form a two-domain regulatory tandem (CBD12). An allosteric Ca(2+) sensor (Ca3-Ca4 sites) is located on CBD1, whereas CBD2 contains a splice-variant segment. Recently, a Ca(2+)-driven interdomain switch has been described, albeit how it couples Ca(2+) binding with signal propagation remains unclear. To resolve the dynamic features of Ca(2+)-induced conformational transitions we analyze here distinct splice variants and mutants of isolated CBD12 at varying temperatures by using small angle x-ray scattering (SAXS) and equilibrium (45)Ca(2+) binding assays. The ensemble optimization method SAXS analysis demonstrates that the apo and Mg(2+)-bound forms of CBD12 are highly flexible, whereas Ca(2+) binding to the Ca3-Ca4 sites results in a population shift of conformational landscape to more rigidified states. Population shift occurs even under conditions in which no effect of Ca(2+) is observed on the globally derived Dmax (maximal interatomic distance), although under comparable conditions a normal [Ca(2+)]-dependent allosteric regulation occurs. Low affinity sites (Ca1-Ca2) of CBD1 do not contribute to Ca(2+)-induced population shift, but the occupancy of these sites by 1 mM Mg(2+) shifts the Ca(2+) affinity (Kd) at the neighboring Ca3-Ca4 sites from ∼ 50 nM to ∼ 200 nM and thus, keeps the primary Ca(2+) sensor (Ca3-Ca4 sites) within a physiological range. Thus, Ca(2+) binding to the Ca3-Ca4 sites results in a population shift, where more constraint conformational states become highly populated at dynamic equilibrium in the absence of global conformational transitions in CBD alignment.
authors.affiliation: Department of Physiology and Pharmacology, Sackler School of Medicine, Tel-Aviv University, Ramat-Aviv 69978, Israel.
authors.forename: Moshe
authors.initials: M
authors.lastname: Giladi
authors.forename: Reuben
authors.initials: R
authors.lastname: Hiller
authors.forename: Joel A
authors.initials: JA
authors.lastname: Hirsch
authors.forename: Daniel
authors.initials: D
authors.lastname: Khananshvili
mesh_terms: D000818:Animals; D001665:Binding Sites; D002118:Calcium / Q000737:chemistry* / Q000378:metabolism; D004285:Dogs; D056004:Molecular Dynamics Simulation; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D019831:Sodium-Calcium Exchanger / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D019831:Sodium-Calcium Exchanger; C110374:sodium-calcium exchanger 1; D002118:Calcium
keywords: Allosteric Regulation; Calcium; Calcium-binding Proteins; Population Shift; Protein Conformation; Sodium-Calcium Exchange